Particularly in motor vehicles, controllers are frequently connected to external components, for example sensors. In this case, a reliable ground connection is required between the circuits in the controller and those in the component. The supply network ground including the conductive parts of the motor vehicle is not suitable for this purpose, owing to the high currents that are present there, and the corresponding interference voltages. Sensor ground lines between controllers and external components are, however, subject to the risk that error voltages, for example in the form of the battery voltage, can be applied to the sensor ground line during installation and removal of the components, or during maintenance and repair work. The protective device according to the invention is intended to provide protection against a hazard such as this to the sensor ground line and to the conductors connected to it in the controller or in the component.